Scientific Forecaster Discussion

000 fxus63 karx 180856 afdarx Area forecast discussion National Weather Service La Crosse WI 256 am CST Mon Feb 18 2019 Short term...(today and tonight) issued at 256 am CST Mon Feb 18 2019 Main short-term fcst concerns are cloud/temperature trends. 06z data analysis had low pressure lifting NE across PA, with a trough extending westward into Iowa. WV imagery showed a mid level circulation moving slowly east over Iowa/southern WI. Convergence/lift from the two and everything in between continued to squeeze out some -sn across much of the south half of the region early this morning. Trend on the -sn was diminishing as the mid level feature moved east. Across the north half of the region, north to NE flow behind the departing features was bringing in some drier air, with a decrease of clouds across much of Minnesota/northern WI. Model runs of 18.00z look to have initialized well. Solutions similar today/tonight as the trough departs and hgts rise across the upper Midwest. Short-term fcst confidence is good this cycle. Column continues to dry today as the sfc-500mb low/trough departs and is replaced by anti-cyclonic sfc-500mb flow. Northerly flow behind these features brings in drier air, with the exception of perhaps some lake moisture/clouds from lk Superior across the NE end of the fcst area. North winds and low level cold advection across the the area today, along with fresh snow cover. Blend of guidance highs mostly 15f to 25f looks good for today. Sfc ridge axis settles over the area tonight, with light/variable/calm wind expected. Some weak 925-850mb warm advection returns overnight but bl should be pretty much decoupled. It would be night for strong radiational cooling over the deep snow cover, however a weak shortwave from the northern plains slides across the area tonight. Per WV/infrared imagery, plenty of mid/high clouds with this feature across the Dakotas/ eastern Montana early this morning, to advect across the area with this feature tonight. Did lower lows few degrees in the normal colder low laying areas along/NE of I-94, otherwise with potential for more of a partly cloudy night the blend of guidance lows a few degrees either side of 0f looked good. Long term...(tuesday through sunday) issued at 256 am CST Mon Feb 18 2019 For Tuesday thru Wednesday night: main forecast concerns this period are the snow chances/amounts centered on Wed and potential impacts. Model runs of 18.00z offering decent agreement at the start of this period as shortwave ridging, ahead of strong troughing/shortwave energy ejecting into the central/Southern Plains, builds across the region Tue. By Tue night/Wed, differences develop as the trough/ shortwave energy lifts NE toward/into the upper Midwest. Models in good agreement for the trough/energy to lift quickly NE of the region Wed night. Even with some detail differences, fcst confidence this period is generally good this cycle. Cold sfc high sits over the area on Tue, with light winds for limited bl mixing under the shortwave ridging and 925-850mb warming. Clouds to already start increasing during the afternoon as the high level moisture ahead of the next trough spreads quickly NE. After a cold start Tue morning, blend of guidance highs again mostly 15-25f look good. Signal remains strong for the next trough/shortwave lifting NE to spread a strong round mid/upper level lift/divergence across the area Tue night/Wed. This with MDT lower level Thermo-dynamic forcing/lift and precipitable water values around 1/2 inch range in the inflow airmass ahead of the approaching sfc-mid level low/trough. Model detail differences Tue night/Wed mainly look to impact timing. End result is 18.00z models remain rather consistent with their 17.12z runs on producing 1/3 to 1/2 inch of quantitative precipitation forecast across the area from mainly 06z Wed to 06z Thu. Dendritic growth zone progged to be around 100mb deep, with some of the stronger upward motion in the column thru it. Snow-water ratios trending more around 15:1 (vs. The 20-30 : 1 sun). A strong consensus for this system to produce mainly 3 to 6 inches of snow across the area centered on Wed. With what looks to be limited potential for greater than 6 inches of snow, will hold off on issuing a Winter Storm Watch at this time. Still a bit early to issue any winter weather advisories, but will continue to refine messaging for an expected 3 to 6 inches of snow. Still some signal for loss of ice nucleation in the cloud tops across the southeast end of the fcst area at times later Wed morning/ afternoon hours. Confidence not the highest on this, but will continue with a mention of some -fzdz in the grids across the southeast end of the fcst area on Wed. For Thursday thru Sunday (days 4 to 7): main fcst concern this period is the potential for another strong winter storm for the weekend. 18.00z medium range model runs in good agreement for rising hgts/SW flow aloft over the region Thu/Fri as then next strong troughing/ energy digs into the SW Continental U.S.. by Sat/sun, plenty of between model and run-to-run detail differences as the energy over the SW Continental U.S. Late Fri ejects into plains and lift NE toward/into the upper Midwest during the weekend (as expected in the day 6-7 timeframe) fcst confidence average to good Thu/Fri then average to below Sat/sun. Sfc high pressure settles over the region for Thu/Fri, under the SW flow aloft. Although not much for sfc-925mb gradients for mixing, the airmass slowly modifies in place thanks to what should be some late Feb sunshine. Blend of guidance highs warming to near normal by Fri continues to look good. Between model and run-to-run detail differences are raising havoc with the timing/strength of the potential system to lift thru the upper Midwest during the weekend. Some consensus among the 18.00z models for a very strong sfc thru mid level system to lift thru the region, bringing a wintry mix to the area starting later Fri night and continuing into sun. However models showing little consistency to their 17.12z runs. With confidence still on the lower side for Sat/sun, for now can do little more than stay with the model/ensemble consensus of mostly 30-60% precip chances and temps near to a bit above normal much of Fri night into sun. && Aviation...(for the 06z tafs through 06z Monday night) issued at 1131 PM CST sun Feb 17 2019 Local radar indicates the light snow is dissipating pretty quickly as the upper level low is almost past the area. This trend will continue into the overnight with some just flurries expected for klse for a couple of hours. Ceilings have already come up to VFR and expect them to remain that way until the clouds scatter out Monday morning as high pressure builds in from the north. && Arx watches/warnings/advisories... WI...none. Minnesota...none. Iowa...none. && $$

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